fix: wrap email preview with full styled header/footer template

The email template preview modal was showing only raw body HTML without
the styled wrapper (green header bar, logo, footer with company name)
that processTemplate() applies when sending. This made preview not match
what recipients actually receive.

Extract wrapEmailHtml() from processTemplate() and reuse it in the
preview endpoint. Also fix logo URL to use FRONTEND_URL consistently.

Closes #229

backend/src/routes/adminEmail.js

@@ -4,6 +4,7 @@ const { body, validationResult } = require('express-validator');
 const { db, logActivity } = require('../database/db');
 const { adminAuth } = require('../middleware/auth');
 const { requirePermission } = require('../middleware/permissions');
+const { wrapEmailHtml } = require('../services/emailProcessor');
 const router = express.Router();
 
 // Get email configuration
@@ -458,9 +459,12 @@ router.post('/templates/:key/preview', adminAuth, requirePermission('email.view'
       });
     }
 
+    // Wrap in the full styled email template with header/footer/logo
+    const wrappedHtml = await wrapEmailHtml(htmlContent, subject, language);
+
     res.json({
       subject,
-      body_html: htmlContent,
+      body_html: wrappedHtml,
       body_text: textContent,
       language
     });